Overfilled bridge structures are frequently formed of precast reinforced four-sided concrete units commonly referred to as arch units, arch culverts, box units or box culverts. As used herein the terminology four-sided bridge unit encompasses all of such structures. The units are used in the case of bridges to support one pathway over a second pathway, which can be a waterway. Four-sided bridge units have a bottom wall structure that facilitates on-site placement with reduced need for foundation preparation.
In the past, the four-sided bridge units of overfilled bridge structures have been constructed with bottom wall structures having a generally planar and continuous top surface and a generally uniform thickness. There is an increasing demand for construction efforts to provide more natural environments and/or to decrease impact on wildlife.
A four-side bridge unit adapted to create a more natural environment through the pathway defined by the bridge units and/or adapted to reduce impact on fish migrations would be desirable.